N scale model trees will range in size as in any layout based on your desires of what you want to duplicate. In general however trees for this scale are going to be between 2 and 4 inches tall. At 1:160 scale a 3 inch tree is equivalent to 40 feet. The great thing about scale trees and armatures is that they will not look bad since they are just a part of the landscape. You can use a variety of sizes in an N scale layout and still have a good looking layout. In fact it is a good idea to use some different sizes so that your layout looks more realistic. After all, how often do you see the exact same height in trees when in nature? You will have trees in all different sizes and variations so presenting such differences in your layout will look more natural.
N scale is one of the smallest scales on the market and is popular because of the ability to put more scenery in a small space. N scale model trees can be a challenge when using natural products just because of their small size. it can be difficult to find a small size that still has a good overall look. Natural wood and plant material will make a far better looking layout however. Sagebrush can be a great option for natural wood because of its unique growth pattern. You can often find tree shaped pieces with sagebrush even in the smaller size range and being a natural material with lots of character you will always end up with a very nice look to your trees. the only other thought you have to consider now is the type of tree you wish to mimic. You have the general differences between deciduous and coniferous but even within these two types of trees you have many varieties that may meet different desires based on the layout you are looking to create.